Method of regulating substrate temperature in a low pressure environment

ABSTRACT

A temperature controlled chuck (20) includes a heating unit (24) and a cooling unit (34). A first cavity (30) separates the heating unit (24) from a wafer substrate (18), and a second cavity (50) separates the cooling unit (34) from the heating unit (24). A first fluid delivery system (60) conducts fluid to the first cavity (30) to facilitate exchanges of heat between the heating unit (24) and the substrate (18). A second fluid delivery system (70) conducts fluid to the second cavity (50) to facilitate exchanges of heat between the heating unit (24) and the cooling unit (34). A control system (90) raises the temperature of the substrate (18) by increasing power to the heating unit (24) and by evacuating fluid from the second cavity (50) and lowers the temperature of the substrate (18) by reducing power to the heating unit (24) and by conducting fluid to the second cavity (50).

RELATED APPLICATIONS

This application is a Division of copending parent application Ser. No.08/560,344, filed Nov. 17, 1995, and entitled TEMPERATURE CONTROLLEDCHUCK FOR VACUUM PROCESSING.

TECHNICAL FIELD

The invention relates to vacuum processing substrates, includingmicroelectronic or optical components, and to temperature control ofsuch substrates in evacuated environments.

BACKGROUND

Vacuum processing operations on substrates, such as deposition of thinfilms, often require control over substrate temperature. Depositionoperations include sputtering, evaporation, and chemical vapordeposition. Related operations such as cleaning, planarizing, annealing,and etching can also require substrate temperature controls.

Some operations require the addition of heat, and others require excessheat to be removed. Also, certain combinations of operations requireboth heating and cooling in various orders. Heating can be accomplishedwith radiant heaters that focus heat energy on substrate surfaces.Cooling is often accomplished by removing the substrates from theevacuated environment. However, with these limited controls, constantsubstrate temperatures are difficult to maintain and temperaturetransitions can be time consuming.

Better control over substrate temperature has been achieved bytransferring heat through substrate supports or chucks. Ordinarily,conduction in a vacuum environment between the substrate and its supportis highly inefficient. However, more efficient transfers of heat arepossible by filling gaps between the substrate and its support with agas that is compatible with the vacuum processing environment.

For example, U.S. Pat. No. 4,909,314 to Lamont, Jr. discloses asubstrate support that also functions as a temperature conditioner suchas a heating unit or a cooling unit. The substrate is clamped to thesupport, and a cavity between the substrate and the support is filledwith a gas such as argon. The gas, which is maintained at a pressureabove that of the vacuum processing environment but well belowatmospheric pressure, transfers heat by convection between the substrateand substrate support.

U.S. Pat. No. 4,949,783 to Lakios et al. discloses a similar substratesupport arranged as a cooling unit. However, instead of merelyconducting gas into a cavity between the substrate and its support, thegas is circulated into and out of the cavity to carry away excess heat.The cooling unit has additional areas exposed to the circulating gas toextract the heat from the gas.

Although the known substrate supports can be arranged to operateeffectively as either heating units or cooling units, some combinationsof vacuum processing operations require both heating and cooling toachieve desired temperature patterns. Substrate supports arranged forheating are slow to cool, and substrate supports arranged for coolingprovide no means for heating. Thus, controlled variations in substratetemperatures are limited to either increases or decreases intemperature, and constant substrate temperatures are difficult tomaintain in processing environments that also involve transfers of heat.

SUMMARY OF INVENTION

Our invention provides improved temperature control over substratesprocessed in evacuated environments. Substrate temperatures can bemaintained more accurately to improve processing quality, andtemperature changes can be made more rapidly to shorten processingtimes.

One example includes the known features of a first temperatureconditioner that supports a substrate in an evacuated environment, afirst cavity between the first temperature conditioner and thesubstrate, and a first delivery system that conducts a fluid to thecavity for exchanging heat between the first temperature condition andthe substrate. However, our invention also includes a secondtemperature, conditioner, a second cavity located between the twoconditioners, and a second delivery system that conducts fluid to thesecond cavity for facilitating exchanges of heat between the twoconditioners.

A control system regulates flows of fluid into and out of the secondcavity to regulate the heat exchanges between conditioners. The heatexchanges are facilitated by the presence of fluid in the second cavityand inhibited by the absence of the fluid. Preferably, the firsttemperature conditioner is a heating unit and the second temperatureconditioner is a cooling unit.

The temperature of the substrate is raised by increasing an amount ofheat produced by the heating unit and by evacuating a fluid from thesecond cavity. The temperature of the substrate is lowered by reducingthe amount of heat produced by the heating unit and by conducting thefluid to the second cavity. The cooling unit, which requires no specialcontrols, extracts excess heat from the heating unit through the fluidin the second cavity. This significantly reduces the amount of timerequired to lower substrate temperature and permits the substrate toremain within the same evacuated environment for more continuousprocessing. The control system can also make smaller changes in fluidpressure in the second cavity, even while heating, to more closelycontrol the substrate temperature.

The first cavity is located within the evacuated environment, but thesecond cavity is isolated from that environment by the first temperatureconditioner. Thus, the delivery of fluid to the first cavity is requiredto support efficient thermal exchanges between the first temperatureconditioner and the substrate, but close contact alone could have beenused to provide efficient heat exchanges between the first and secondtemperature conditioners. Instead, our invention provides for separatingthe two conditioners by an independently evacuatable space (i.e., thesecond cavity) to inhibit as well as facilitate the heat exchanges byvarying the fluid pressure.

DRAWINGS

FIG. 1 is a schematic layout in side cross section of a sputterdeposition system including our new temperature controlled chuck.

FIG. 2 is a more detailed side cross-sectional view of the temperaturecontrolled chuck.

FIG. 3 is an enlarged fragmentary view of a portion of FIG. 2 showing aseal between two temperature conditioners.

FIG. 4 is a more detailed bottom view of a two-zone heating unit thatcan be used as one of the temperature conditioners.

DETAILED DESCRIPTION

A sputter deposition system 10 incorporating our invention is depictedin FIG. 1. The system 10 includes the usual features of a vacuum chamber12, which is evacuated by a pump 14, and a target 16 of material to bedeposited on a wafer substrate 18. An electrical potential is applied tothe target 16 in the presence of an ionizable gas, causing gas ions tostrike the target 16 and release atoms of the target material into aplasma that deposits the target material on the wafer substrate 18.

The plasma can be controlled by magnets (not shown) located in thevicinity of the target 16 or the wafer substrate 18. For example,coassigned U.S. Pat. No. 5,248,402 discloses an apple-shaped magnetronmounted in the vicinity of a target for more evenly eroding the target,and coassigned U.S. application Ser. No. 08/369,381 discloses a magneticorienting device in the vicinity of a wafer substrate for magneticallyorienting the deposited target material. Both references are herebyincorporated by reference. Further details of sputtering systems arefound in these references. The invention, however, is also applicable toother vacuum processing operations including evaporation, chemical vapordeposition, planarizing, annealing, etching, and cleaning.

A new substrate support, namely chuck 20, for use in such vacuumprocessing operations is shown also in FIG. 2 in a less schematicformat. The wafer 18 mounts on an annular seat 22 of a heating unit 24,which is preferably made with a stainless steel body 26 (such asINCONEL) supporting a heating element 28 made of aniron-aluminum-chromium alloy (such as KANTHAL) for heating wafers up to1000 degrees Centigrade. The annular seat 22 forms, together with thewafer 18 and the heating unit body 26, a first cavity 30. An annularclamp 32 secures the wafer 18 to the seat 22.

A cooling unit 34, which is preferably made with a stainless steel body36 containing coolant passageways 38, is positioned adjacent to theheating unit 24. As best seen in FIG. 3, the stainless steel body 36 ismade from two plates 36a and 36b. The passageways 38 are formed aschannels in the plate 36b and are covered by the plate 36a.

An annular rim 42 of the heating unit 24 engages an O-ring seal 44mounted in an annular seat 46 of the cooling unit 34. An annular clamp48 urges the annular rim 42 toward the annular seat 46 for compressingthe O-ring seal 44. The O-ring seal 44 is preferably made fromperfluoroelastomer (such as KALREZ) to withstand elevated temperatures.The annular rim 42 is made as thin as possible to limit conduction ofheat to the O-ring seal 44. On the other hand, the annular clamp 48,which is preferably made from copper, is made much thicker to conductheat from the annular rim 42 to the cooling unit 34.

The O-ring seal 44 and annular seat 46, together with the respectivebodies 26 and 36 of the heating and cooling units 24 and 34, form asecond cavity 50, which is evacuatable independently of the first cavity30 and the vacuum chamber 12. Another annular clamp 52 and annular seal54, shown in FIGS. 1 and 2, secure the body 36 of the cooling unit 34 toa body 56 of the chuck 20. The seal 44 isolates the second cavity 50from the evacuatable space of the vacuum chamber 12, and the seal 54separates the vacuum chamber 12 from ambient atmospheric conditionswithin the chuck 20.

Separate delivery systems 60 and 70 convey fluids to the first andsecond cavities 30 and 50. The delivery system 60 includes a tank 62 orother source of compressed gas, such as argon, which is compatible withthe processing operations within the vacuum chamber 12. A conduit 64conveys the gas through the chuck body 56, the cooling unit body 36, andthe heating unit body 26 into the first cavity 30. Along the conduit 64,a flow control valve 66 limits the flow of gas to the first cavity 30,and a vacuum gauge 68 monitors pressure in the first cavity 30.

The delivery system 70 also includes a tank 72 or other source ofcompressed gas. However, since the second cavity 50 is isolated from thevacuum chamber 12, a wider variety of gases can be used including gasesexhibiting better convection qualities such as helium, hydrogen, andnitrogen. A conduit 74 conveys the gas through the chuck body 58 andinto a fitting 80 that extends through the cooling unit body 36 into thesecond cavity 50. Flows from the tank 62 to the second cavity 50 arelimited by flow control valve 76. A vacuum gauge 78 monitors pressure inthe second cavity. In addition, a vacuum pump 84 is connected to theconduit 74 for evacuating fluid from the second cavity 50.

A control system 90 includes a processor 92 with inputs 94 and 96 fromthe two vacuum gauges 68 and 78 and outputs 98, 100, and 102 to the twoflow control valves 66 and 76 and the vacuum pump 84 for regulatingfluid pressure in the first and second cavities 30 and 50. The firstcavity 30 is evacuated together with the vacuum chamber 12. A desiredfluid pressure is maintained in the first cavity 50 by conducting fluidto the cavity 30 at a rate that compensates for any leakage between theannular seat 22 of the heating unit 24 and the wafer substrate 18. Thevacuum pump 84 is controlled in conjunction with flow control valve 76for regulating fluid pressure in the second cavity 50.

The heating unit 24 is controlled by lead wires 104 that extend throughthe fitting 80 into the second cavity 50, where they are curled in ahorizontal plane to accommodate expansion and contraction of the heatingelement 28. Referring again to FIG. 3, the second cavity 50 has a height"H" that is at least equal to a diameter "D" of the lead wires 104 toprovide the necessary space for curling the lead wires 104. Preferably,the height "H" is at least 2 millimeters.

The temperature of the wafer 18 can be approximated by monitoring thetemperature of the heating unit body 26 and by calculating anapproximate temperature offset of the wafer 18 based on predeterminedrates of energy transfer between the heating unit 24 and the wafer 18.The wafer temperature could also be monitored directly within the vacuumchamber 12, such as by using an optical sensor (not shown) to detectlevels of radiated heat.

The chuck 20 is supported on a vertical drive 108 for raising andlowering the wafer substrate 18 within the vacuum chamber. Bellows 110seal the chuck body 56 to the vacuum clamber 12 while permitting therelative vertical movement of the chuck 20. Preferably, a plurality ofbellows (not shown) provides separate exits from the vacuum chamber 12for the conduits 64 and 74.

During both heating and cooling, the flow control valve 66 preferablymaintains a predetermined pressure (e.g., 5 to 20 Torr) in the firstcavity 30 sufficient to provide thermal communication between theheating unit 25 and the wafer substrate 18 and coolant is preferablycirculated through the coolant passages 38 of the cooling unit 34.During heating, power is supplied to the heating unit 24 and the vacuumpump 84 evacuates fluid from the second cavity 50 to inhibit transfersof heat from the heating unit 24 to the cooling unit 34. During cooling,the power supplied to the heating unit 24 is reduced (or terminated) andthe flow control valve 76 maintains fluid pressure (e.g. 10 to 50 Torror more) in the second cavity 50 sufficient to promote transfer of heatfrom the heating unit 24 to the cooling unit 34.

The wafer substrate 18 can be maintained at a more stable elevatedtemperature by slightly overpowering the heating unit 24 (i.e., byproducing excess heat) and by maintaining fluid pressure in the secondcavity 50 to remove the excess heat. The wafer temperature can beslightly raised (e.g., 10 degrees Centigrade) by lowering fluid pressurein the second cavity 50 and slightly lowered (e.g., 10 degreesCentigrade) by raising the fluid pressure in the second cavity 50. Thisprovides faster and more accurate control over wafer temperatures.

An alternative two-zone heating unit 114 is shown in FIG. 4 as it wouldappear when viewed from the second cavity 50. The heating unit 114 has athermally conductive body 116 supporting inner and outer heatingelements 118 and 120. The inner heating element 118 is powered throughlead wires 122, and the outer heating element 120 is powered throughlead wires 124. Two thermocouples 126 and 128 monitor temperatures atdifferent radial positions in the conductive body 116. The twothermocouples 126 and 128 and the lead wires 122 and 124 emerge from thesecond cavity 50 through the lifting 80 similar to the lead wires 104 ofthe heating unit 24.

The two-zone heating unit 114 can be used together with the cooling unit34 to compensate for radial temperature variations in the wafersubstrate 18. For example, heat is often dissipated mole rapidly from aperiphery of the wafer 18 than from its center. To compensate for thisradial temperature variation, more power can be applied to the outerheating element 120 than to the inner heating element 118. However,conductivity tends to diminish the radial temperature gradient in theheating unit body 116, so zone control of the heating unit 114 alone maynot be sufficient to maintain a uniform temperature distributionthroughout the wafer substrate 18.

According to our invention, fluid pressure in the second cavity 50 canbe controlled to conduct heat from the heating unit body 116, requiringone or more of the heating elements 118 and 120 to be over powered tomaintain the desired overall temperature of the wafer substrate 18.Increasing the fluid pressure in the second cavity 50 tends to sustaintemperature gradients in the heating unit body 116 induced by differentzonal heating, and decreasing the fluid pressure in the second cavity 50tends to diminish this effect. Thus, control over both the fluidpressure in the second cavity 50 and the power to the different heatingelements 118 and 120 provides for controlling both the overalltemperature of the wafer substrate 18 and the temperature distributionthroughout the wafer substrate 18.

More or differently shaped heating zones could be used to affect othertemperature distributions in the wafer substrate 18. The second cavity50 could also be divided into independently controllable zones (notshown) for further influencing temperature gradients. The cooling unit34 could also be controlled such as by regulating the flow ortemperature of the circulating coolant. These and many other changes orenhancements will be apparent to those of skill in art in accordancewith the overall teachings of this invention.

We claim:
 1. A method of maintaining a substrate at a uniformly elevatedtemperature in an evacuated processing environment comprising the stepsof:mounting the substrate on a conductive body within a vacuum chamberin thermal communication with a heating unit that is divided into aplurality of separately controlled heating elements; exposing thesubstrate to processing resulting in a more rapid dissipation of heatfrom one part of the substrate than another; conducting a flow of fluidinto an evacuatable cavity separating the heating unit from a coolingunit for withdrawing some of heat generated by the heating unit;controlling both fluid pressure in the cavity and power applied to twoof the separately controlled heating elements to sustain an overallelevated temperature of the substrate; and overpowering one of theheating elements while increasing fluid pressure in the cavity tosustain a more uniform temperature distribution throughout the substratethan would be possible at a lower fluid pressure in the cavity withoutraising the overall temperature of the substrate.
 2. The method of claim1 in which said step of controlling includes adjusting the fluidpressure over a continuum of pressures for sustaining the overalltemperature of the substrate.
 3. The method of claim 1 in which heatdissipates more rapidly from a periphery than a center of the substrate,the heating elements include inner and outer heating elements, and saidstep of overpowering includes overpowering the outer heating element. 4.The method of claim 1 in which said step of controlling includessupplying different amounts of power to the heating elements.
 5. Themethod of claim 1 in which said step of controlling fluid pressureincludes raising fluid pressure to reduce the overall substratetemperature.
 6. The method of claim 1 in which said step of controllingfluid pressure includes lowering the fluid pressure to increase theoverall substrate temperature.
 7. The method of claim 1 including thefurther step of dividing the evacuatable cavity between the heating andcooling units into separately controllable zones for further influencingthe temperature distributions in the substrate.